The present invention generally relates to safety devices for power driven tools and machine tool systems, and more paticularly, to an anti-kickback power tool control method and system which is capable of sensing a significant and rapid change in the load on the tool and turning off the power to the tool and/or applying a brake before lock-up occurs between the tool or implement and the workpiece. As used herein, the term "implement" means the tool or implement which is driven by the motive power source of the power tool, Thus, the term "implement" includes such devices as saw blades or chains, drill and router bits, abrasive wheels and the like. The terms "power tool", "machine tool" and similar terms refer to the total combination including the motive power source and the driven implement.
Kickback is defined as that condition when the power driven tool or other implement abnormally and rapidly engages the workpiece in such a manner as to transfer at a high rate the power of the motive power source and/or the inertial energy of the power tool to the power tool or the workpiece or both resulting in an undesired and generally uncontrolled motion of the power tool or the workpiece or both. Kickback can be caused by a variety of factors including but not limited to sudden or excess force by the operator, nonuniform hardness or a defect in the workpiece, and where the workpiece is wood or other cellular material, the accumulation of moisture. In the case of a saw, the kerf may close and pinch the blade to cause a kickback, and in the case of a drill, kickback may occur when the bit breaks through the workpiece.
Consider, for example, a portable circular saw of the type used on construction sites. Typically, plywood sheathing is first nailed to a framing structure, such as roofing trusses, and then the projecting ends of the plywood are sawed off evenly. If in this process a kickback were to occur due to the kerf closing as the ends of the wood sag under their own weight, the portable circular saw could kick out of the workpiece and fly back toward the operator, possibly resulting in serious bodily injury. On the other hand, in the case of a bench saw or a radial saw where the workpiece is fed into the saw blade, a kickback would result in the workpiece being driven back toward the operator at a high rate of speed, again with the possibility of serious bodily injury. It will be understood by those familar with the art that kickback is not a condition which is limited to circular saws but may be experienced with any power driven tool or machine tool system. For example, kickback may occur with portable, bench or stationary power drills, routers and shapers, portable and bench planers, abrasive wheel grinders, milling machines, reciprocating saws and the like. All of these tools may be typically driven with electrical motors, including universal, D.C. single phase or polyphase motors, but kickback is not a condition dependent on the motive power source but rather on the abnormal engagement of the tool or other implement with the workpiece as defined above. Thus kickback is a potential hazard with any cutting power tool whether driven by an electric motor, a pneumatic motor, an internal combustion engine or any other motive power source. In fact, kickback is an especially dangerous hazard in chain saws which are most typically driven by small internal combustion gasoline engines.
There have been many attempts to eliminate or reduce the hazards of kickbacks in power tools. For example, in bench saws and in radial arm saws, a plurality of freely rotating pawls carried on an adjustable support with the support clamped to the saw bench or frame been provided. In use, the pawls are adjusted downwardly toward the workpiece so that the workpiece as it is fed to the cutting edge of the saw blade pushes the pawls upwardly and freely passes under the pawls. In the event of the kickback, the pawls are designed to dig into the surface of the workpiece to prevent the workpiece from being kicked back toward the operator. This anti-kickback device is generally effective, but may damage the workpiece.
In the case of chain saws where kickback is especially dangerous, two approaches are currently in use. The first approach is bases on the determination that most kickbacks occur when the workpiece is engaged by the cutting chain as it traverses the end of the guide bar. To prevent this from happening, some chain saws have been equipped with a protective cover for the cutting chain at the extreme end of the guide bar. The second approach to anti-kickback protection in a chain saw employs a chain brake designed to be actuated when a kickback occurs. Typically, the chain brake is operated by the deflection of a lever which hits the operator's hand due to motion of the saw in kickback.